Confluence of complex surface impedance and critical current studies of model defects in niobium films near critical temperature*

Studying and manipulation of vortex matter is key to understanding fundamental physics of superconducting materials, improving the properties of superconducting devices, and developing new concepts for superconductor applications. Two major approaches to experimentally characterize vortex dynamics are to measure (i) the critical current density or (ii) surface impedance as a function of applied field. In this work we combine both approaches to characterize thin 200 nm Nb films used in superconducting electronic circuits. We measured the vortex pinning constant (2 kN/m^2), viscous drag (~1e-8 N-s/m^2), and depinning frequency (~ 10 GHz) near the critical temperature (~0.85Tc) using a Parallel Plate Resonator technique. In addition, we measured the depairing critical currents (~ 0.75 MA/cm^2 at 0.9Tc) and the pinning force density (~1 MN/m^3 at 0.989Tc) as a function of temperature near Tc. The test results agree well with vortex wiggling experiments using a Scanning SQUID Microscope, and our numerical simulations of vortex potentials and vortex attempt frequencies based on the Ginzburg-Landau model and Kramers theory of escape rate of a Brownian particle from a potential well, respectively.